Multilayer ceramic circuit board

ABSTRACT

A method for producing a multilayer ceramic circuit board including the steps of forming a multilayer structure consisting of patterns of copper-based paste and glass-ceramic layers, the glass-ceramic layers consisting of a mixture of 10 percent to 75 percent by weight of α-alumina, 20 percent to 60 percent by weight of crystallizable or noncrystallizable glass which can be sintered at a temperature lower than the melting point of copper, and 5 percent to 70 percent by weight of quartz glass, based on the total weight of the glass-ceramic, blended with a binder containing a thermally depolymerizable resin; prefiring the multilayer structure in an inert atmosphere containing water vapor, the partial pressure of which is 0.005 to 0.3 atmosphere, at a temperature where the thermally depolymerizable resin is eliminated; and firing the multilayer structure in an inert atmosphere containing no water vapor at a temperature below the melting point of copper so as to sinter the glass-ceramic.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing a multilayerceramic circuit board, and more particularly a method for producing acircuit board comprising a glass-ceramic having a low dielectricconstant and copper conductors having a low electrical resistivity,enabling high-speed signal transmission.

2. Description of the Related Art

A multilayer circuit board consists of layers of electrical insulators,usually ceramic, and metallic conductors. The ceramic must have a lowdielectric constant as well as a high electrical resistivity, highbending strength, low thermal expansion coefficient, and high thermalconductivity. The metallic conductor is preferably copper due to its lowelectrical resistivity and price. During the firing of the multilayerboard, the organic binder included in the ceramic must be decomposedwithout any residual carbon and the copper must not be oxidized.

U.S. Pat. No. 4,234,367 to Herron et al., assigned to IBM, discloses amethod of making multilayer glass structures having an internaldistribution of copper-based conductors by firing in a controlledambient atmosphere of H₂ : H₂ O=10⁻⁴ to 10⁻⁶.5, at a temperature belowthe melting point of copper. In this method, β-spoduemene or cordieriteis preferably used as the crystallizable glass. It is, however,difficult to control the ambient atmosphere, due to the extraordinarilysmall amount of the hydrogen content.

U.S. Pat. No. 4,504,339 to Kamehara et al., assigned to Fujitsu Limited,discloses a method for producing a multilayer glass-ceramic structurehaving copper-based conductors therein for use as a circuit board. Inthis method, a multilayer structure comprises layers of a thermallydepolymerizable resin and glass-ceramic containing preferably 40 percentto 60 percent by weight of Al₂ O₃ and layers of a copper-based paste.The structure is fired in an inert atmosphere containing water vapor,the partial pressure of which is 0.005 to 0.3 atmosphere, preferably at550° C. to 650° C. The structure is then sintered in a nitrogenatmosphere containing no water vapor, preferably at about 900° C.However, if the firing temperature in the inert atmosphere containingwater vapor is higher than 650° C., the residual carbon is trapped inthe closed pores in which H₂ O vapor is present. The carbon then reactswith the H₂ O to form CO₂. This phenomenon results in bloating of theglass-ceramic.

Japanese Unexamined Patent Publication (Kokai) No. 59-11700 to Ogiharaet al., assigned to Hitachi, discloses a multilayer ceramic circuitboard obtained by firing a mixture of silica and a noncrystallizableglass in an atmosphere containing nitrogen, hydrogen, and water vapor.However, the bending strength of the board is inevitably lower than thatof one formed with crystalline ceramics.

SUMMARY OF THE INVENTION

It is an object of the present invention to produce a multilayer ceramiccircuit board having only a very small amount of residual carbon fromthe binder of the ceramic, even after firing at a high temperature.

It is another object of the present invention to produce a multilayerceramic circuit board employing ceramic having a high bending strengthand a low thermal expansion coefficient.

It is still another object of the present invention to produce amultilayer ceramic circuit board having low water absorption and lowsurface roughness.

It is yet another object of the present invention to produce amultilayer ceramic circuit board having a low dielectric constant, a lowelectrical resistivity, and a low delay time without reducing thedielectric strength.

According to the present invention, there is provided a method forproducing a multilayer ceramic circuit board comprising the steps offorming a multilayer structure having patterns of copper-based paste andglass-ceramic layers, the glass-ceramic layers consisting of a mixtureof 10 percent to 75 percent by weight of α-alumina, 20 percent to 60percent by weight of crystallizable or noncrystallizable glass (the term"noncrystallizable glass" or "glass" as used in this specificationexcludes quartz glass) which can be sintered at a temperature lower thanthe melting point of copper, and 5 percent to 70 percent by weight ofquartz glass, based on the total weight of the ceramic, blended with abinder containing a thermally depolymerizable resin; prefiring themultilayer structure in an inert atmosphere containing water vapor, thepartial pressure of which is 0.005 to 0.3 atmosphere, at a temperatureat which the thermally depolymerizable resin is eliminated; and firingthe multilayer structure in an inert atmosphere containing no watervapor at a temperature below the melting point of copper so as to sinterthe glass-ceramic.

It is desirable that the crystallizable glass be cordierite orspoduemene.

It is preferable that the noncrystallizable glass be borosilicate glassor aluminosilicate glass.

It is advantageous that the thermally depolymerizable resin bepolymethacrylate ester, polytetrofluoroethylene, poly-α-methylstyrene,or a mixture thereof.

It is useful that the prefiring be carried out in two steps, at 350° C.to 450° C. in the first step and at 650° C. to 900° C. in the secondstep, and that the final firing be carried out at a temperature higherthan 900° C. and lower than 1083° C.

The multilayer structure may be formed by means of the laminated greensheet technique or the multilayer screen printing technique and exhibitsthe same mechanical and electrical properties after firing regardless ofthe forming procedure.

BRIEF EXPLANATION OF THE DRAWINGS

The present invention will be described in more detail below withreference to the appended drawings, in which:

FIG. 1 show the relationship between the dielectric constant and theamount of glass added;

FIG. 2 shows the relationship between the residual carbon and amount ofglass added;

FIG. 3 shows the relationship between the bending strength and theweight ratio in an α-alumina/quartz-glass/glass system;

FIG. 4 shows the relationship between density and the amount of glassadded in an α-alumina/glass system;

FIG. 5 shows the relationship between the dielectric strength andresidual carbon;

FIG. 6 shows the weight ratio in an α-alumina/quartz-glass/glass system;

FIG. 7 shows the relationship between residual carbon and the prefiringtemperature;

FIG. 8 shows the prefiring and firing profile;

FIG. 9 is a plan view of a signal layer of a multilayer circuit board;

FIG. 10 is an exploded side view of a layer structure of a copperconductor multilayer circuit board;

FIG. 11 shows the relationship between the electric sheet resistivity ofthe copper conductor and the firing temperature;

FIG. 12 shows the relationship between the fired density and firingtemperature;

FIG. 13 shows the change of fired density as a function of the firingtemperature and firing time;

FIG. 14 shows the relationship between water absorption and the firingtime;

FIG. 15 shows the relationship between surface roughness and the firingtime;

FIG. 16 shows the relationship between water absorption and the fireddensity; and

FIG. 17 shows the relationship between surface roughness and the fireddensity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally, a circuit board should have a high bending strength, a lowdielectric constant, and a high softening point and, thus, a low amountof residual carbon.

Usually, a glass-ceramic board is obtained by firing a mixture ofα-alumina and noncrystallizable glass or crystallizable glass(hereinafter referred to as just "glass") blended with an organicbinder. It is important that α-alumina and glass exhibit oppositedielectric constant, bending strength, and softening pointcharacteristics, as shown in Table 1.

The glass component is useful in sintering the glass-ceramic andlowering the dielectric constant. Glass, however, lowers the softeningpoint, which leads to an increase in the amount of residual carbon.

                  TABLE 1                                                         ______________________________________                                                   α-alumina                                                                       Glass      Quartz glass                                    ______________________________________                                        Dielectric constant                                                                        10.0      4 to 8     3.8                                         Softening point (°C.)                                                               mp 2015   ca. 700 to 900                                                                           ca. 1500                                    Bending strength                                                                           ca. 7000  ca. 400 to 700                                                                           ca. 600                                     (kg/cm.sup.2)                                                                 ______________________________________                                    

According to the present invention, a certain amount of quartz glasswith a low dielectric constant and a high softening point is added tothe mixture of α-alumina and glass so as to lower the dielectricconstant (FIG. 1) and raise the softening point. This makes it possibleto eliminate the binder at a high temperature and compensates for theunfavorable effect of the glass, i.e., decreases the amount of theresidual carbon (FIG. 2).

Addition of too large an amount of quartz glass, which has a low bendingstrength, however, would lower the bending strength of theglass-ceramic, which contains α-alumina and glass.

The relationship between the bending strength and weight ratio in anα-alumina/quartz-glass/glass system (FIG. 3) is obtained from thefollowing general formula of three components. ##EQU1## σ: Bendingstrength w: Weight ratio

ρ: Density

A ceramic circuit board should exhibit a bending strength of up to 1000kg/cm². Thus, the weight ratio of α-alumina must be at least 10 percent.

Generally, a glass-ceramic increases in density when the constituentamount of glass is increased (FIG. 4). The figure shows the density ofan α-alumina/quartz-glass/borosilicate-glass system, sintered at 1050°C. for two hours.

A circuit board should have a density of at least 90 percent, so it isnecessary to add at least 20 percent by weight of glass, such asborosilicate glass.

Residual carbon results from the carbon material in closed pores formedin a glass-ceramic prior to elimination of the binder. This will beclear from the previous description, i.e., the addition of a largeamount of glass lowers the sintering temperature of the glass-ceramicand increases the amount of residual carbon (FIG. 2). The increase ofresidual carbon leads to a lower dielectric strength. In order tomaintain the dielectric strength at a desirable value, the residualcarbon should be as low as 100 ppm (FIG. 5). Turning back to FIG. 2, theglass-ceramic should have up to 60 percent by weight of glass.

In summary, as to the weight ratio of the three components of theglass-ceramic, there should be at least 10 percent by weight ofα-alumina and 20 percent to 60 percent by weight of glass. Theselimitations fall in the hatched area of FIG. 6. Consequently, the upperlimit of the weight ratio of α-alumina is determined as 75 percent. Theupper limit of the quartz glass is determined as 70 percent from thebalance of the sum of the lower limits of α-alumina and glass. The lowerlimit of quartz glass is determined to be 5 percent because sufficienteffects are not obtained with quartz glass of under 5 percent.

The resin used to bind the glass-ceramic should be a thermallydepolymerizable resin, preferably polymethacrylate ester,polytetrafluoroethylene, poly-α-methylstyrene, or a mixture thereof. Themultilayer ceramic circuit board, which comprises, prior to firing,layers of ceramic blended with one of these thermally depolymerizableresins and patterns of copper-based paste, is prefired in an inertatmosphere containing water vapor, the partial pressure of which is0.005 to 0.3 atmosphere, at a temperature at which the thermallydepolymerizable resin is eliminated and thereafter is subjected to finalfiring in an inert atmosphere containing no water vapor at a temperaturebelow the melting point of copper, at which the glass-ceramic issintered.

The prefiring step may comprise two steps for eliminating the binderresin in the inert atmosphere containing water vapor. In a first step,the binder resin is thermally depolymerized at a temperature lower than500° C. In the second step, the residual organic substance which was notdepolymerized in the first step reacts with water vapor at a temperaturehigher than 500° C.

A glass-ceramic composition including α-alumina, quartz glass, andborosilicate glass, each in an amount of about 33 percent, based on thetotal weight of glass-ceramic, was subjected to prefiring. Therelationship of the first prefiring temperature, predetermined as 280°C. to 480° C. for 8 hours, the second prefiring temperature,predetermined as 600° C. to 925° C. for 8 hours, the amount of residualcarbon, and the chemical and physical behavior of copper wasinvestigated. The results are shown in FIG. 7. The numerals plotted inthe figure represent the amount of residual carbon. Their contour linesare drawn between the plotted points having equivalent numerals.

The first prefiring step produces minimal residual carbon at atemperature between 350° C. to 450° C., while the second prefiring stepproduces less than 100 ppm of residual carbon at a temperature of 650°C. or more. On the other hand, copper diffuses in the glass-ceramicstructure at a temperature higher than 450° C. in the first prefiringstep, and oxidation of copper is observed at a temperature higher than900° C. in the second prefired step.

Therefore, the elimination of the thermally depolymerizable resin ispreferably carried out at temperature of 350° C. to 450° C. in the firstprefiring step and 650° C. to 900° C. in the second prefiring step.

After eliminating the thermally depolymerizable resin, the glass-ceramicstructure is heated in an inert atmosphere without water vapor at atemperature lower than the melting point of copper, thereby sinteringthe glass-ceramic and thus increasing the density of the structure. Theprefiring and firing profile is shown in FIG. 8.

The silica glass-containing glass ceramic circuit board according to thepresent invention exhibits excellent mechanical and electricalproperties, i.e., a high bending strength, a low thermal expansioncoefficient, a low residual carbon level, a high fired density, a lowwater absorption, a low dielectric constant, and a short delay time andcan use a copper conductor having a low electrical resistivity. Forexample, the fired density may be 99 percent, the water absorption 0.05percent, and the dielectric constant one-half and the delay timetwo-thirds of those of a conventional α-alumina circuit board.

The present invention will now be further illustrated by way of examplesand comparative examples, which by no means are meant to limit the scopeof the invention.

EXAMPLE 1

A glass-ceramic multilayer structure was prepared by the multilayerscreen printing process. α-alumina (Alcoa A-14), quartz glass (Corning7913), and borosilicate glass (Corning 7740) were mixed each in a weightratio of 29.3 percent, i.e., each about 33 percent based on the totalweight of glass-ceramic, and blended with a binder comprising 8.7percent by weight of polymethacrylate ester resin and 3.3 percent byweight of dibutyl phthalate to form a glass-ceramic green sheet.

Signal layers were produced by printing copper paste (ESL 2310), to formconductor patterns, on green sheets (0.3×150×150 mm) formed by thedoctor blade technique. In the green sheets, copper balls (0.2 mmφ) werevertically embedded to form via-holes. Thirty layers were laminated andpressed with a force of 25 MPa at 130° C. for 30 minutes.

The multilayer structure was then subjected to prefiring and finalfiring. The firing conditions and the mechanical and electricalproperties of the obtained glass-ceramic and copper are shown in Table2.

The mechanical and electrical properties of the glass-ceramic were foundto depend on those of the components before mixing, rather than theanalytical composition of the obtained glass-ceramic. This is becausethe α-alumina and quartz glass neither melt nor soften at firingtemperatures below the melting point of copper and maintain theiroriginal properties even after the firing and also the amount of addedα-alumina and quartz glass controls the properties of the obtainedglass-ceramic.

                                      TABLE 2                                     __________________________________________________________________________                    Examples                                                                      1    1a   1b   2    2a                                        __________________________________________________________________________    Components (wt %)                                                             α-alumina 34   50   50   34   50                                        Quartz glass    33   0    0    33   0                                         Borosilicate glass                                                                            33   50   50   0    0                                         β-spoduemene                                                                             0    0    0    33   50                                        Prefiring                                                                     First                                                                         Temperature (°C.)                                                                      400  none 400  400  none                                      Time (h)        8    none 8    8    none                                      Second                                                                        Temperature (°C.)                                                                      800  650  800  800  700                                       Time (h)        8    4    8    8    4                                         Firing                                                                        Temperature (°C.)                                                                      1010 950  950  1010 990                                       Time (h)        8    2    2    8    2                                         Density (%)     99   96   87   98   96                                        Residual carbon (ppm)                                                                         35   80   60   30   70                                        Dielectric strength (kV/cm)                                                                   50   50   --   --   --                                        Dielectric constant (/MHz)                                                                    4.9  5.6  4.8  5.3  6.0                                       Delay time (ps/cm)                                                                            75   82   --   --   --                                        Bending strength (kg/cm.sup.2)                                                                1800 2000 1200 1800 2000                                      Thermal expansion coefficient X                                               10.sup.6 (1/°C.)                                                                       3.0  4.5  --   3.5  7.0                                       Copper conductor                                                                              --   --   Bro- --   --                                                                  ken                                                 Impedance (Ω )                                                                          95   96   --   --   --                                        __________________________________________________________________________

COMPARATIVE EXAMPLES 1a and 1b

The same procedure was carried out as in Example 1, except that thecomposition was modified to be 50 percent by weight of α-alumina and 50percent by weight of borosilicate glass, based on the total weight ofthe glass-ceramic, and the firing conditions were changed. The obtainedglass-ceramic circuit board of Example 1a exhibited a dielectricconstant higher than that of Example 1; on the other hand, the obtainedglass-ceramic circuit board of Example 1b exhibited a dielectricconstant lower than that of Example 1, and the reliability (such asstrength or water absorption) is degraded by the low density as reportedin Table 2. Therefore, the ceramic of Example 1b is not deemed suitablefor use as a ceramic circuit board.

EXAMPLE 2

The same procedure was carried out as in Example 1, except thatβ-spodumene was added instead of borosilicate glass. The glass-ceramicexhibited properties like those in Example 1.

COMPARATIVE EXAMPLE 2a

The same procedure was carried out as in Example 2, except that quartzglass was omitted from the composition and the firing conditions weremodified. The obtained glass-ceramic circuit board exhibited adielectric constant higher than that of Example 2.

The method for producing a multilayer ceramic board according to thepresent invention results in a lower dielectric constant without anincreased amount of residual carbon and in addition results in improvedreliability of the board and increased speed of signal transmission.

EXAMPLE 3

A green sheet of 0.34±0.01 mm thickness was produced by the doctor-bladetechnique from a slurry of a glass-ceramic mixture, having an averageparticle size in the range of 2 to 6 microns, of α-alumina (Alcoa A-14),quartz glass (Corning 7913), and borosilicate glass (Corning 7740),blended with a binder comprising an acrylic resin and di-butyl phthalatein a weight ratio shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Component of SiO.sub.2 Glass Added Glass-Ceramic                              Component           Weight percent                                            ______________________________________                                        α-Alumina (ALCOA A-14)                                                                      25.7                                                      Borosilicate glass (Corning 7740)                                                                 25.7                                                      Silica glass (Corning 7913)                                                                       25.7                                                      Acrylic resin       15.3                                                      Di-butyl phthalate  7.6                                                       ______________________________________                                    

Minute copper balls were filled through the green sheet to formvia-holes. A copper paste was printed on the surface of the green sheetto form a copper conductor pattern. A signal layer is shown as anexample in FIG. 9, in which the dots represent via-holes and thestraight lines represent signal patterns. In the same way as the signallayer, various patterns, i.e., a power supply, a ground, and aninput/output pattern, were printed on corresponding green sheets, asshown in FIG. 10. These green sheets were laminated under 25 MPa at 130°C. to form an 11-layered laminate. Needless to say, these green sheetsmay be laminated to form much more than 11 layers, e.g., may belaminated to form 30 layers.

The produced quartz glass-containing α-alumina borosilicateglass-ceramic laminates were fired at various temperatures for variousperiods in an atmosphere of nitrogen containing less than 5 ppm ofoxygen and less than 5 ppm of water vapor in order to determine theoptimum range of firing conditions for obtaining the desirablemechanical and electrical performances of a circuit board withoutdisadvantageous effects on the copper conductor.

FIG. 11 shows the relationship between the sheet resistivity of a copperconductor with a width and a thickness of 0.8 mm and 0.025 mm,respectively, formed on an internal ceramic layer, and temperature, asfired for 30 minutes. The sheet resistivity is lowest at 1.1 m Ω/□ at afiring temperature of 950° C. to 1000° C. and increases up to 1.2 m Ω/□at higher temperatures. The patterned circuit broke after firing at themelting point of copper. Therefore, the firing temperature is desirablynot higher than 1083° C. of the melting point of copper.

FIG. 12 shows the relationship between the fired density of a firedgreen sheet laminate of 2.72 mm thickness and firing periods of varioustemperatures. The thickness was reduced to about two-thirds of theoriginal by the firing.

FIG. 13 shows curves obtained from the same data plotted in FIG. 12. Thefired density was calculated from the following equation:

    FD(%)=Gs/Dt×100

where Gs is the bulk density and Dt is the theoretical density. The bulkdensity was determined according to Japanese Industrial Standard (JIS) C2141. The fired density reaches a maximum value in a short firingperiod. The higher the temperature, the shorter that firing period. Thefired density reaches 99 percent after 8 hours at 1010° C., 4 hours at1020° C., or 2 hours at 1040° C.

FIGS. 14 and 15 show the water absorption and the surface roughness whenthe green sheet was produced from a slurry of a glass-ceramic mixture ofα-alumina, quartz glass, and borosilicate glass having an averageparticle size of 3 microns, respectively, after firing at varioustemperatures and various periods. The surface roughness Rmax wasdetermined according to JIS B 0601 by means of a surface roughnessmeter.

As can be seen from FIGS. 16 and 17, the fired density has an intimaterelationship with the water absorption and the surface roughness. Thesurface roughness, expressed as Rmax, falls between the two curved linesdrawn in FIG. 17. A fired density of more than 98 percent corresponds toless than 0.05 percent of water absorption and less than 1 micron ofsurface roughness Rmax.

Thus, the firing condition of a quartz-glass containing glass-ceramic isdetermined in the range of 1010° C. to 1040° C. for 2 to 8 hours,preferably at 1010° C. for 8 hours.

EXAMPLE 4

An 11-layered circuit structure of 2.2 mm thickness was produced bylaminating green sheets, each having copper conductor layers thereon, at130° C. under a pressure of 25 MPa, by prefiring at 800° C. for 8 hoursin a nitrogen atmosphere containing water vapor so as to remove theblended binder comprising an acrylic resin and di-butyl phthalate, andthereafter by firing at 1010° C. for 8 hours in a nitrogen atmospherewithout water vapor.

A 30-layered circuit structure of laminated green sheets, each having acopper conductor layer thereon, of about 6 mm was produced by theabove-mentioned method.

The electrical properties of the fired copper conductor multilayeredceramic circuit boards according to the present invention are shown inTable 4 below, in comparison with two conventional circuit boards, onean α-alumina board having a molybdenum conductor II and the other anα-alumina-borosilicate glass ceramic board having a gold conductor III.

                  TABLE 4                                                         ______________________________________                                        Electrical Properties of Multilayered Circuit Boards                                          I      II      III                                            ______________________________________                                        Electrical resistivity (Ω/cm)                                                             0.15     0.8     0.7                                        Dielectric constant                                                                             4.9      9.8     5.6                                        Dielectric strength (kV/mm)                                                                     50       --      50                                         Delay time (ps/cm)                                                                              75       112     82                                         Impedance (Ω)                                                                             95       65      96                                         ______________________________________                                    

We claim:
 1. A sinterable precursor material for a glass-ceramicinsulating layer of a multilayer ceramic circuit board made up of copperconductors separated by glass-ceramic insulating layers, said materialcomprising a mixture of 10 percent to 75 percent by weight of α-aluminaparticles, 20 percent to 60 percent by weight of crystallizable ornon-crystallizable glass which has a softening point in the range ofapproximately 750° C. to 900° C. and is sinterable at a temperaturelower than the melting point of copper, and 5 percent to 70 percent byweight of quartz glass particles based on the total weight of themixture.
 2. A precursor material as set forth in claim 1 wherein saidcrystallizable glass comprises cordierite or spoduemene.
 3. A precursormaterial as set forth in claim 1 wherein said non-crystallizable glasscomprises borosilicate glass or aluminosilicate glass.
 4. A precursormaterial as set forth in claim 1 comprising a mixture of approximately34 weight percent of α-alumina, 33 weight percent of borosilicate glassand 33 weight percent of quartz glass.
 5. A precursor material as setforth in claim 1 comprising a mixture of approximately 34 weight percentof α-alumina, 33 weight percent of β-spoduemene and 33 weight percent ofquartz glass.
 6. A green sheet for a glass-ceramic layer of a multilayerceramic circuit board comprising an admixture of the precursor materialof claim 1 and a binder.
 7. A green sheet for a glass-ceramic layer of amultilayer ceramic circuit board comprising an admixture of theprecursor material of claim 2 and a binder.
 8. A green sheet for aglass-ceramic layer of a multilayer ceramic circuit board comprising anadmixture of the precursor material of claim 3 and a binder.
 9. A greensheet for a glass-ceramic layer of a multilayer ceramic circuit boardcomprising an admixture of the precursor material of claim 4 and abinder.
 10. A green sheet for a glass-ceramic layer of a multilayerceramic circuit board comprising an admixture of the precursor materialof claim 5 and a binder.
 11. A sinterable composite comprising aplurality of copper conductor layers separated by layers formed from theprecursor material of claim
 1. 12. A sinterable composite comprising aplurality of copper conductor layers separated by layers formed from theprecursor material of claim
 2. 13. A sinterable composite comprising aplurality of copper conductor layers separated by layers formed from theprecursor material of claim
 3. 14. A sinterable composite comprising aplurality of copper conductor layers separated by layers formed from theprecursor material of claim
 4. 15. A sinterable composite comprising aplurality of copper conductor layers separated by layers formed from theprecursor material of claim
 5. 16. A multilayer ceramic circuit boardprepared by firing the composite of claim 11 in an inert atmospherecontaining no water vapor at a temperature below the melting point ofcopper.
 17. A ceramic circuit board comprising a layer of conductivematerial supported by a ceramic insulating layer, said board having beenprepared by admixing the precursor material of claim 1 with a binder;forming a layer from the admixture; laying a conductive material on thelayer to form a composite; subjecting the composite to conditionssuitable for removing the binder; and thereafter firing the compositeunder temperature conditions suitable for causing the crystallizable ornon-crystallizable glass to sinter without causing sintering of theα-alumina and quartz glass components.
 18. A fired ceramic circuit boardcomprising a layer of a conductive material supported by a ceramicinsulating layer, said insulating layer comprising 10 percent to 75percent by weight of α-alumina particles, 20 percent to 60 percent of asintered glass matrix composed of glass component which initially had asoftening point in the range of approximately 750° C. to 900° C. and wassinterable at a temperature lower than about 1083° C., and 5 percent to70 percent by weight of quartz glass particles, said circuit boardhaving been fired in an inert atmosphere containing no water vapor andat a temperature above the sintering temperature of the glass componentand below the sintering temperatures of α-alumina and quartz glass so asto produce a structure comprising α-alumina and quartz glass particlessurrounded by said glass matrix.
 19. A fired ceramic circuit boardcomprising a conductive layer supported by a ceramic insulating layer,said insulating layer comprising 10 percent to 75 percent by weight ofα-alumina particles, 5 percent to 70 percent by weight quartz glassparticles, and 20 to 60 percent by weight of a matrix of a sinteredglass component surrounding said particles.
 20. A fired ceramic circuitboard as set forth in claim 19 wherein said ceramic insulating layercontains less than about 100 ppm residual carbon.